Control of electrical drives requires determining the position and speed of a rotor. There are several ways to determine such parameters. First, the position of the rotor may be determined by an array of phototransistors and a special shutter coupled to the rotor shaft, or by using Hall-effect sensors. Such systems are described in T. Kenjo, Electrical Motors and Their Controls, Oxford University Press, (1994), 176 pp. Second, the speed signal may be obtained by using a small permanent magnet tachometer generator, attached to the drive, or by using magnetic or optical sensors generating pulses for each angular increment of the rotor. Such systems are described in W. Leonhard, Control of Electrical Drives, 2.sup.nd ed., Springer (1966), 420 pp. Third, a resolver may determine the position of the rotor by a two-phase (sine/cosine) signal at a carrier frequency modulated sinusoidally by the rotation of the rotor. Such a system is described in J. R. Hendershot, Jr. and T. Miller, Design of Brushless Permanent-Magnet Motors, Magna Physics Publishing (1994), p. 1-19. These methods require precise mechanical placement of sensors or mechanical contact between moving parts.
It would be advantageous to deliver signals to or from the rotating parts of electrical machines without either mechanical or galvanic contact and complex sensor supporting systems.